<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #app{
            width: 200px;
            height: 200px;
            background: pink;


            position: absolute;
            left: 0;
            top: 0;
            right: 0;
            bottom: 0;
            margin: auto;
        }

        #inner{
            width: 100px;
            height: 100px;
            position: absolute;
            bottom: -10px;
            right: -10px;
            background: black;
        }
    </style>
</head>
<body>
    <div id="app">
        xxxx
        <div id="inner">

        </div>
    </div>
</body>
</html>